Chapter 2 - Thermodynamics & Psychrometrics

Chapter 2: Thermodynamics and Psychrometrics

Overview

• This chapter reviews thermodynamics principles applied to air-conditioning and refrigeration.

• Chapters 3 and 2 in the ASHRAE Handbook provide further details on psychrometrics and refrigeration cycles.

2.1 Fundamental Concepts and Principles

2.1.1 Thermodynamics

• Definition: Study of energy, its transformations, and interactions with matter.

  • Key elements: Energy and entropy.

  • Energy: Capacity for doing work, categorized as stored or transient.

  • Entropy: Measure of molecular disorder; higher entropy means greater disorder.

• Laws of Thermodynamics:

  • First Law: Energy cannot be created or destroyed (Conservation of energy).

  • Second Law: Entropy in an isolated system always increases; every energy transformation incurs energy loss for work.

• Rudolf Clausius: Introduced entropy to quantify available energy loss in transformations.

2.1.2 Forms of Energy

• Based on the conservation of energy law, energy transferred as heat and work are important concepts.

• Heat (Q): Energy transferred due to temperature difference; considered energy in transit.

• Work (W): Transfer of energy due to force exerted through a distance.

2.1.3 Stored Forms of Energy

• Stored forms include:

  • Internal Energy (U): Energy possessed due to molecular motion/position; includes kinetic and potential energy.

    • Kinetic internal energy: Related to velocity and temperature.

    • Potential internal energy: Related to molecular forces and phase.

  • Potential Energy (PE): Energy due to an object's position in a gravitational field.

  • Chemical Energy (E): Energy due to atomic arrangements.

  • Nuclear Energy (Eᴀ): Energy due to binding forces in atomic nuclei.

2.1.4 Transient Forms of Energy

• Heat: Transfer mechanism where energy moves from hot to cold areas.

  • Positive heat: Energy transferred to a system.

  • Negative heat: Energy transferred from a system.

• Work: Energy transferred due to pressure differences across a boundary.

2.1.5 System and Surroundings

• System: Defined object or quantity of matter isolated for study.

• Surroundings: Everything external to the system.

• Boundaries can be fixed or movable, real or imaginary.

2.1.6 Properties and State

• Properties: Observable characteristics like temperature, pressure, specific volume, internal energy, enthalpy, and entropy.

• State: Condition of a system distinguished by properties.

  • Equilibrium state: No spontaneous changes unless external conditions change.

  • Types of equilibrium: Thermal, mechanical, and chemical.

2.1.7 Processes and Cycles

• Process: Change in state characterized by interaction across system boundaries. Defining terms include:

  • Isobaric: Constant pressure.

  • Isothermal: Constant temperature.

  • Adiabatic: No heat exchange.

• Cycle: A series of processes returning to initial states; all properties identical in first and last states.

2.1.8 Reversibility

• Reversible Process: Can be reversed without changing the surrounding conditions.

  • Irreversible processes are natural and cannot be reversed without external influence or changes.

2.2 Laws of Thermodynamics in More Depth

2.2.1 Second Law Applications

• The second law restricts the first law by stating that energy transformations have limits; it fundamentally defines efficiency.

• Thermal efficiency of heat engines and performance of heat pumps can be evaluated using:

  • Kelvin-Planck Statement: No cyclic device can convert heat completely into work.

  • Clausius Statement: No process can solely result in heat transfer from a cooler to a hotter body.

2.3 Properties of a Substance

2.3.1 Specific Volume and Density

• Specific volume: Volume per unit mass.

• Density: Mass per unit volume; inversely related to specific volume.

2.3.2 Pressure

• Pressure: Force per unit area exerted by fluids.

• Measurement: Differences often measured relative to atmospheric pressure.

2.4 Psychrometrics

• Definition: Study of moist air properties and behavior.

• Components of Psychrometry: Dry air, moisture content, pressure, and temperature relationships.

2.4.1 Moist Air Properties

• Saturation occurs when vapor and liquid coexist at equilibrium temperatures and pressures.

• Charts show relationships between various psychrometric properties for analysis and solutions in HVAC processes.

2.5 Refrigeration and Heat Pumps

2.5.1 Vapor Compression Refrigeration Cycle

• Core components of refrigeration cycles include evaporators, compressors, condensers, and expansion valves.

• The basic cycle involves heat absorption and rejection, showing energy transformation via phase changes of refrigerant.

2.5.2 Heat Pumps

• Operate by moving heat from low to high temperature, often using the refrigeration cycle in reverse.

• Coefficient of performance defines efficiency in heating applications.

2.5.3 Absorption Refrigeration

• Utilizes an absorbent fluid to reduce work requirements, relying on heat transfer rather than mechanical compression.

2.6 References and Further Reading

• Provides numerous sources and literature relevant to thermodynamics and psychrometrics including ASHRAE handbooks and engineering texts.